In general, in order to assure that a compressed component has mechanical strength and resistance to disintegration or crumbling, the material used is a powder with an irregular grain shape, characterized by a substantial specific surface area with respect to the grain size. The mechanical interengagement of the grains in the compacting operation assures that the component has a sufficient level of mechanical strength for all the handling operations required in respect of the compressed component. Powders with an irregular grain shape are generally produced by a water atomization process. This process comprises spraying a jet of liquid metal with one or more jets of water under pressure. The powder is projected into a water bath for concluding cooling thereof. After settling, the powder is dried and subjected to a first de-oxidation or reduction operation. The powder produced has a grain size which for the major part is less than 160 microns. The shape of the grains depends on the composition and the conditions of the atomization operation, which is generally fairly turbulent. On the other hand, as the powder is required to be subjected to a reduction step, its composition in respect of alloy elements must be limited to those whose oxides can be easily reduced. For example, for steels which are substantially non-alloyed, the typical composition AISI 4600 contains 2% of nickel and 0.8% of molybdenum. Chromium-manganese compositions which are less expensive can be produced only at the expense of a burdensome treatment for reduction using carbon at a temperature of 1200.degree. C., followed by crushing of the sintered powder.
The process for atomization by means of a neutral gas makes it possible totally to avoid this problem in regard to composition. Such a process comprises spraying a jet of liquid metal with a plurality of jets of gas under pressure. Solidification of the droplets and cooling of the powder take place in a confined chamber; the spraying gas which fills the chamber is neutral with respect to the atomized metal. Depending on the composition used, the gas may be either argon or nitrogen. The atomization chamber and apparatus may be advantageously designed on the basis of the principles of French Pat. Nos. 73-43159 or 73-45788. The grain size of the resulting powder is between a few microns and 500 microns, with a spheroidal grain shape. The oxygen content which varies according to the composition is typically of the order of from 100 to 200 ppm, and the specific surface area, with respect to the grain size, is close to the minimum value of a quasi-spherical powder.
In regard to such spheroidal powders, the mechanical strength of the compressed components depends on the inter-particle contact surface produced in the compacting operation. In practice, it is found that, for a Vickers hardness under a load of 500 g, which is better than 100, the mechanical strength and resistance to crumbling of the compressed parts are at insufficient levels to permit normal handling of such compressed parts, which severely limits the potential uses of such powders for producing blanks by cold compacting.